This invention pertains to the working combination of a vehicle inching mechanism and a braking mechanism, interlocked in such a way that the inching and braking operations overlap without making it impossible to start or stop the vehicle by inching.
The vehicle inching system has been known which comprises an inching valve for the on-off control of hydraulic fluid pressure activating a clutch built into a planetary gear transmission. The intermittent engagement of the clutch results in the vehicle movement, in either a forward or reverse direction, in small increments. It has also been known and practiced to start or stop the vehicle at highly reduced speed by gradually increasing or decreasing the fluid pressure until the clutch becomes half engaged.
The inching mechanism is usually interlocked with the braking mechanism of the vehicle. According to the prior art the brake system has been held unactivated during inching operation and applied after clutch disengagement by the inching operation. A problem arose here because of the time lag existing from the moment of clutch disengagement to that of brake application. Being unrestrained during this time lag, the vehicle has been prone to make unexpected travel under its own weight and/or by inertia.
An obvious solution to this problem is to interlock the inching and the braking mechanisms in such a manner that braking takes place before completing declutching. Some requirements must be met in carrying this solution into practice, however. One is that inching and braking operations overlap so as to allow the low-speed start and stop of the vehicle by the inching control. For example, in bringing the vehicle to a stop at low speed, the braking force should gradually increase while the clutch pressure is on a decrease. In starting the vehicle at low speed, on the other hand, the clutch pressure should increase in step with a gradual decrease in braking force.
Another requirement is that the operator, as he depresses the inching pedal to any degree, be enabled to discern whether the clutch pressure is greater than the braking force or vice versa in that pedal posittion. The operator should also be enabled to sense the inching pedal position where the actual clutch capacity equals the braking force. This is because the desired low-speed start and stop of the vehicle take place by gradually releasing or depressing the inching pedal in the vicinity of that position.